Noise reduction of traveling-wave tubes by circuit refrigeration



Oct. 12, 1965 s. BLOOM 3,211,947

NOISE REDUCTION OF TRAVELINGWAVE TUBES BY CIRCUIT REFRIGERATION Filed May 14, 1962 I I 1 I I I I z I 1', 1 I I I INVENTOR, STANLEY BLOOM.

ATTORNE United States Patent NOISE REDUCTION OF TRAVELING-WAVE TUBES BY CIRCUIT REFRIGERATION Stanley Bloom, Plainfield, N.J., assignor to the United States of America as represented by the Secretary of the Army Filed May 14, 1962, Ser. No. 194,738 4 Claims. (Cl. 315-32) The present invention relates to traveling-wave tubes, and more particularly to means for cooling travelingwave tubes to obtain a reduction in noise.

Traveling-wave tubes employing various types of cooling devices have heretofore been proposed. The cooling process has been accomplished by a variety of methods. Some tubes have been provided with a jacket which carries a coolant, while others have employed a helix which comprises a hollow tube through which the coolant flows. In other tubes the coolant flows through helix support rods, through the slow wave structure or down the middle of traveling-wave tube.

There are a number of motives for cooling a travelingwave tube. When the power handling capacity of a traveling-wave tube is increased to a certain level, it is necessary to provide a coolant to dissipate the heat. In order to reduce the amplitude of reflected waves, which occur in some traveling-wave tubes due to mismatches in the microwave circuit thereof, lossy material is introduced into the microwave circuit to attenuate the reflections. Again it becomes necessary to provide a coolant to dissipate the heat generated in the lossy material. Still another reason for cooling a traveling-wave tube is to reduce the thermal noise due to circuit loss. The latter reason is the one with which this invention is mainly concerned.

The thermal noise due to circuit loss sets a lower limit on the noise factor of beam-type microwave amplifiers. Means will be described herein for reducing the effect of such thermal noise by circuit refrigeration.

The usual expression for the minimum noise factor of the traveling-wave tube only takes account of the noise contributed by the electron beam; viz.

Here (in standard notation) T is cathode temperature; S, 1r and S are beam noise parameters; x is the circuit gain parameter and d is the circuit loss parameter. The circuit loss, d, enters into the above formula only because of the effect of loss on gain.

Circuit loss can, however, be an :active contributor to the noise factor. This arises because the circuit loss generates Nyquist or thermal noise. When this noise is introduced, the new minimum noise factor formula reads where T is the circuit temperature. The derivation of this equation is contained in the article, Effect of Distributed- Loss Noise Generators on Traveling-Wave-Tube Noise Factor by S. Bloom, RCA Review, June 1961, vol. XXII, No. 2, pp. 347 to 349. It can therefore be seen that circuit refrigeration reduces the noise factor by reducing T and by reducing a, which is a function of T.

One of the previously mentioned known methods of cooling a traveling-wave tube is to provide the tube with a jacket which carries a coolant. Cooling jackets which have been used prior to this invention have had the disadvantage of being confined to a region on the tube envelope between the input and output couplers since the input and output waveguides could not be efficiently coupled through the cooling jacket. Therefore, the entire length of the tube could not be cooled and maximum noise reduction could not be obtained.

3,211,947 Patented Oct. 12, 1965 It is therefore an object of this invention to provide a traveling-wave tube with a jacket which extends along the entire length of the tube.

It is another object of this invention to provide a cooling jacket for a traveling-wave tube having means for coupling the input and output energy through the jacket.

It is a further object of this invention to provide a traveling-wave tube having low thermal noise.

These and other objects, which will become more apparent from the following description, are achieved by providing a traveling-wave tube with a cooling jacket along its entire length, the jacket being provided with annular metal discs for the purpose of coupling input and output energy through the jacket.

The invention will be discussed in connection with the following drawings, wherein:

FIGURE 1 is a cross-sectional view of a portion of a traveling-wave tube constructed in accordance with this invention; and

FIGURE 2 is a cross-sectional view of the fluted-glass helix support member.

Referring now to FIGURES 1 and 2, there is shown a portion of a helix-type traveling-wave tube having a helix 1 supported by a fluted-glass envelope 2 which is designed so that good thermal contact exists between the helix and the glass. Envelope 2 is supported within cooling jacket 3 and forms an annular coolant chamber therewith. A coolant 4 flows through this chamber. A coupler 5 is situated outside the cooling jacket and is adapted to couple energy between the helix and external circuits.

Due to the specific arrangement of this'invention the coolant chamber extends over the complete length of the helix, with the couplers being outside of the coolant chamber. The fact that the couplers are uncommonly far from the electron beam is compensated for by the use of annular metal rings 6, 7, 8 and 9 which are located within the coolant chamber. Rings 6 and 8 are located at the inner surface of cooling jacket 3 and are aligned with the open portion of coupler 5 while rings 7 and 9 are located at the outer surface of envelope 2 and are substantially concentric with rings 6 and 8 respectively.

Thus there has been described a cooling arrangement for traveling-wave tubes which efficiently cools the helix over its entire length while permitting efficient coupling of the input and output couplers through the cooling chamber by insertion of annular metal rings within the chamber. This arrangement provides more efficient cooling of the tube, and therefore more effectively reduces the thermal noise due to circuit loss.

What is claimed is:

1. In a traveling-wave tube having a tube envelope and a slow-wave structure supported within the tube envelope and in thermal contact therewith, the improvement comprising: a cooling jacket surrounding said tube envelope, the coolant chamber formed by said jacket and said envelope extending over the entire length of said slow wave structure; a waveguide coupler located outside said cooling jacket; and conductive means within the chamber formed by said jacket and said envelope for efiiciently coupling said coupler with said slow-wave structure through the chamber formed by said jacket and said envelope, said conductive means comprising a plurality of concentric rings of different diameters. located between said waveguide coupler and said slow wave structure.

2. The traveling-wave tube of claim 1 in which two of said rings are located on the inner surface of said cooling jacket and two of said rings are located on the outer surface of said tube envelope.

3. A traveling-wave tube having low thermal noise comprising: a fluted-glass envelope; a helix supported within said envelope; a cooling jacket surrounding said envelope along the entire length of the helix and forming a cooling chamber with said envelope; a microwave coupler located outside said jacket; and conductive means within said cooling chamber for etficiently coupling said coupler with said helix through said cooling chamber, said conductive means comprising a plurality of concentric rings of different diameters located between said microwave coupler and said helix.

4. A traveling-wave tube comprising: a fluted-glass envelope; a helix supported within said enevlope; a cooling jacket surrounding said envelope along the entire length of said helix and forming a cooling chamber with said envelope; at microwave coupler located outside said jacket; a first pair of conductive annular rings located at the inner surface of said jacket and aligned with said coupler; and a second pair of conductive rings located at the outer surface of said envelope substantially concentric with said first pair of rings.

References Cited by the Examiner UNITED STATES PATENTS ROBERT SEGAL, Primary Examiner. 

1. IN A TRAVELING-WAVE TUBE HAVING A TUBE ENVELOPE AND A SLOW-WAVE STRUCTURE SUPPORTED WITHIN THE TUBE ENVELOPE AND IN THERMAL CONTACT THEREWITH, THE IMPROVEMENT COMPRISING; A COOLING JACKET SURROUNDING SAID TUBE ENVELOPE, THE COOLANT CHAMBER FORMED BY SAID JACKET AND SAID ENVELOPE EXTENDING OVER THE ENTIRE LENGTH OF SAID SLOW WAVE STRUCTURE; A WAVEGUIDE COUPLER LOCATED OUTSIDE SAID COOLING JACKET; AND CONDUCTIVE MEANS WITHIN THE CHAMBER FORMED BY SAID JACKET AND SAID ENVELOPE FOR EFFICIENTLY COUPLING SAID COUPLER WITH SAID SLOW-WAVE STRUCTURE THROUGH THE CHAMBER FORMED BY SAID JACKET AND SAID ENVELOPE, SAID CONDUCTIVE MEANS COMPRISING A PLURALITY OF CONCENTRIC RINGS OF DIFFERENT DIAMETERS LOCATED BETWEEN SAID WAVEGUIDE COUPLER AND SAID SLOW WAVE STRUCTURE. 